Methods of making electric heating units



Nov. 7, 1961 E. w. BARNES 3,007,236

METHODS OF MAKING ELECTRIC HEATING UNITS Filed Feb. 27, 1956 INVENTOR.Emma/2 W Barnes United States Patent 3,007,236 METHODS OF MAKINGELECTRIC HEATING UNITS Emmett W. Barnes, Lombard, llL, assignor toGeneral Electric Company, a corporation of New York Filed Feb. 27, 1956,Ser. No. 567,849 4 Claims. (Cl. 29--155.67)

The present invention relates to methods of making electric heatingunits of the sheathed resistance conductor type.

I-Ieretofore, it has been proposed that in manufacturing an electricheating unit of the sheathed resistance conductor type, an assembly beproduced that includes an elongated metallic sheath enclosing both anelongated helical resistance conductor and an elongated member formed ofmagnesium metal, as well as a charge or packing of crystalline magnesiumoxide or other electrical insulating and heat-conducting material, andthat the assembly then be subjected to heat treatment in an autoclave inorder to oxidize in situ the member formed of magnesium metal toamorphous magnesium oxide, with the resulting expansion of the compositemass of refractory material enclosed in the sheath so as to obtain thedesired additional compacting thereof in place between the helicalresistance conductor and the sheath in the finished heating unit.

While the proposal has obvious advantages in principle, the difiicultytherewith has resided in the suggested details of the steps that arecarried out in the autoclave to achieve the oxidation of the magnesiummetal. More particularly, the assembly mentioned is placed in theautoclave and first subjected to the action of high temperature steam ata gauge pressure of 1000 p.s.i. for a time interval of about 12 hours,thereby to obtain complete hydration of the magnesium metal with theresulting production of magnesium hydroxide. Next, the assembly issubjected to the autoclave to a high temperature baking-out action invacuum throughout a time interval of several days in order to obtainsubstantial dehydration of the magnesium hydroxide with the resultingproduction of amorphous and substantially anhydrous magnesium oxide.

of making an electric heating unit of the sheathed resistance conductortype, and involving an improved and simplified step for converting insitu at least the outer skin of a member formed essentially of magnesiummetal into anhydrous amorphous magnesium oxide, while the member isembedded in a porous packing of refractory material enclosed in themetallic sheath of the heating unit.

A further object'of the invention is to provide a method of making anelectric heating unit of the sheathed resistance conductor type, andinvolving an improved and simplified step for converting in situ amember formed essentially of a metal selected from the group consistingof beryllium, manganese, aluminum and titanium into the correspondingmetal oxide, while the member is embedded in a porous packing ofrefractory material enclosed in the metallic sheath of the heating unit.

A further object of the invention is to provide a method of making anelectric heating unit of the sheathed resistance conductor type thatinvolves a simple autoclave step that may be positively and safelycarried out in a simple manner, within a reasonable time interval.

3,00?,Z3fi Patented Nov. 7, 1561 ICC A still further object of theinvention is to provide an improved process of the character describedin which the member formed essentially of magnesium metal is oxidizeddirectly with gaseous oxygen at a modest gauge pressure and at acontrollable elevated temperature and without danger of ignition of themember.

Further features of the invention pertain to the particular arrangementof the steps of the method, whereby the above-outlined and additionaloperating features thereof are attained.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the accompanying drawing, in which:

FIGURE 1A is an enlarged fragmentary view, partly in section, of anassembly that is employed in making an electric heating unit of thesheathed resistance conductor type in accordance with the method of thepresent invention;

FIG. 1B is an enlarged fragmentary view, partly in section, of thefinished electric heating unit that has been produced from the assemblyof FIG. 1A;

FIG. 2A is an enlarged fragmentary view, partly in section, of amodified form of an assembly that is employed in making an electricheating unit of the type noted in accordance with the present method;

FIG. 2B is an enlargedfragmentary view, partly in section, of thefinished electric heating unit that has been produced from the assemblyof FIG. 2A; and

FIG. 3 is a perspective view of an electric heater, actually the innercoil of an electric hotplate, that has been produced from the finishedelectric heating unit of either FIG. 1B or FIG. 2B.

Referring now to H6. 18 of the drawing, the electric heating unit 10there illustrated and made in accordance with the method of the presentinvention fundamentally comprises an elongated tubular metallic sheath11 that may be formed of a suitable nickel-chromium-iron alloy andhaving a substantially circular cross-section, and an elongated helicalelectrical resistance conductor or element 12 that may be formed of asuitable nickel-chromium alloy and located substantially centrallywithin the sheath 11 and embedded in a body of heat-conducting andelectrical-insulating material of composite structure including acentral core portion 313 formed of amorphous magnesium oxide and anouter surrounding tubular portion 14 formed of c 'stalline magnesiumoxide,

Also, the unit 10 comprises a pair of elongated conductive terminals 15and 16 respectively arranged in the opposite ends of the sheathv 11 andrespectively elec trically connected at the inner ends thereof to theopposite ends of the resistance conductor 12. More particularly, theterminal 15 is provided with a shoulder adjacent to the inner endthereof defining an inwardly projecting stem-like section 15a to whichseveral turns of the resistance conductor 12 are suitably secured at theadjacent end thereof, as by welding, or the like. The outer end of theterminal 15 projects from the adjacent outer end of the sheath 11 and issealed in place by an insulating plug 17 formed of glass, or the like.The construction and arrangement of the terminal is is identical to thatof the terminal 15; whereby the terminals 15 and 16 are disposed alongthe longitudinal axis of the unit 10 and afford electrical connectionsto the resistance conductor 12 for the heating purpose.

In manufacturing the electric heating unit 10, shown in FIG. 1B, firstan assembly is produced of the character of that shown in FIG. 1A; andspecifically, the helical resistance conductor 12 is first wound upon anelongated rod 18 formed essentially of magnesium metal; and the oppositeends of the resistance conductor 12 are suitably secured to the stems15a and 15:1 respectively provided upon the inner ends of the terminals15 and 16. This subassembly of the resistance conductor 12, themagnesium metal rod 13 and the terminals 15 and 16 is arrangedsubstantially centrally within the tubular sheath 11; and one end of thesheath 11, such, for example, as the lefthand end thereof, is suitablyclosed by a tubular insulating plug 59 surrounding the intermediateportion of the terminal is and by a fiber backing washer 2% arranged inthe end of the sheath 11 in surrounding relation with the intermediateportion of the terminal 15 and exteriorly of the plug Then the extremeleft-hand end of the sheath ill is bent over, as indicated at Ma, inorder to retain the washer 249 in place.

At this time, the assembly is transferred to a loading machine of thecharacter of that disclosed in US. Patent No, 2,316,659, granted onApril 13, 1943 to John L. Andrews; the terminal 16 being arranged at thetop of the loading machine and being held in place by a hook, not shown;whereby the resistance conductor 32 is in a position depending from theterminal 16 and disposed substantially centrally within the upstandingtubular sheath lit. The loading machine is then operated in aconventional manner, whereby the charge 14 of finely divided crystallinemagnesium oxide is introduced into the upper end of the sheath 11adjacent to the terminal 36 and into embedding relation with respect tothe resistance conductor 12, the magnesium metal rod 18 and the innerends of the terminals 15 and 16. As the charge 14 of refractory materialis introduced into the sheath 11, it is tamped or vibrated into place toprovide a firm, but porous, packing filling the space between thecentrally disposed elements l2, l8, l and it? and the surroundingtubular sheath ll.

After filling of the sheath 11, the assembly is removed from the loadingmachine mentioned and the upper or right-hand end of the sheath lll, asshown in FIG. 1A, is closed by an insulating plug 21 and an associatedfiber backing washer, not shown, the extreme right-hand end of thesheath 11 being bent over to retain the backing Washer in place, asindicated at lib. Also, the extreme outer end of the terminal 16 thatcooperates with the hook provided in the loading machine is severed. Atthis time, the assembly of FIG. 1A is produced, the righthand endthereof being identical to the left-hand end thereof, as illustrated;and in passing, it is mentioned that the plugs 1% and 21 may be of thefrangible insulating type consisting essentially of a chalk-likematerial of porcelain character. However, it is noted that while theplugs 19 and 21 retain the charge 14 in place, they are not sealed inair-tight relation to the opposite ends of the sheath 1i, and they arenot sealed in air-tight relation to the respective intermediate portionsof the terminals and 16.

At this time, the assembly is transferred to a rolling machine of thecharacter of that disclosed in US. Patent No. 2,677,172, granted on May4, 1954 to Sterling A. Oakley; and the rolling machine is operated inorder to subject the assembly to a preliminary compacting step, theassembly being moved upwardly, while supported in an upright position,through the several rolling passes. Specifically, in the preliminaryrolling operation, the diameter of the sheath 11 is modestly reduced soas modestly to reduce the cross-sectional area of the charge 14 and soas to crush the frangible plugs 19 and 21 in order to obtain thecorresponding modest compacting of the refractory mate-rial arranged inthe sheath 11 and supporting the elements l2, l3, l5 and 16substantially centrally thereof. At this time, following the preliminaryrolling step, the charge of refractory material 14, as well as therefractory material resulting from the crushing of the frangible plugs19 and 21, is still porous, as explained more fully below.

Following the preliminary rolling step, the assembly, havingsubstantially the appearance, as illustrated in FIG.

1A, is transferred to an autoclave of conventional constructionincluding a heating chamber, an associated electric heater, a connectingvacuum pump, a connecting oxygen tank provided with a pressureregulator, pressure gauges and suitable valves and fittings, all of aconventional character.

In accordance with the method, the heating chamber of the autoclave isclosed after the assembly is loaded therein; and actually the heatingchamber is arranged to receive a substantial group or number of theassemblies, as a matter of production facility. After closing andsealing of the heating chamber, it is evacuated and the vacuum is heldfor about 15 minutes so as to remove air from the porous packing ofrefractory material enclosed in the sheath ll. Thereafter, gaseousoxygen is introduced into the heating chamber under gauge pressure ofabout p.s.i., and the assembly is soaked therein for about 15 minutes.Again the heating chamber is evacuated and the vacuum is held for about15 minutes so as to remove residual air from the porous packing ofrefractory material enclosed in the sheath ll'l.

Thereafter gaseous oxygen is again introduced into the heating chamberunder gauge pressure of about 70 p.s.i., and this pressure is maintainedtherein in order again to permeate the packing of refractory materialand to contact the magnesium metal, rod 18. The temperature of theheating chamber is then elevated to a reaction temperature disposedbelow the ignition temperature of magnesium metal at the gauge pressureof about 70 p.s.i., which elevated reaction temperature is maintainedfor a suflicient time interval completely to react the magnesium metalrod 18, so as completely to convert the same to anhydrous amorphousmagnesium oxide; whereby the mass of amorphous magnesium oxidecomprising the core 13 is produced in the assembly, as indicated in FIG.IE.

it has been discovered that under the reaction conditions specified, theignition temperature of magnesium metal is about 1085 F, whereby thereaction temperature is maintained in the heating chamber at about 1075F, this reaction temperature being safely below the ignition temperatureof magnesium metal, yet sufliciently close thereto to insure a high rateof reaction. It is estimated that in the manufacture of the heating unit10, the required time interval of the reaction is about 8 hours;however, the assembly is retained in the autoclave under the reactionconditions set forth for a time interval of about 16 hours so aspositively to insure the complete conversion of the rod 18 frommagnesium metal to magnesium oxide. Specifically, this time interval isnot critical, provided it is sufliciently long to insure the completereaction of all of the magnesium metal and the complete conversionthereof to amorphous magnesium oxide; i.e., the maintenance of thereaction conditions in the heating chamber of the autoclave for sometime interval following the complete conversion of the magnesium metalto amorphous magnesium oxide is in nowise deleterious since the reactionis automatically terminated when the conversion is complete.

After the complete reaction of the magnesium met-a1 rod 18 has beenachieved in the autoclave, the heating chamber thereof is opened to theatmosphere and allowed to cool; whereupon the assembly is removed fromthe heating chamber and again transferred to the rolling machine;whereupon it is subjected to a final compacting step, similar to thepreliminary compacting step previously described, but substantially moresevere. In the final rolling operation, the diameter of the sheath 111is substantially reduced so as substantially to reduce thecross-sectional area of the composite body of refractory material,including the core 13 of amorphous magnesium oxide and the surroundingcharge 14 of crystalline magnesium oxide, so as to produce a highlycompacted dense composite body of refractory material embedding thehelical resistance conductor 12 and retaining the same in place inspaced-apart relation with respect to the sheath 11. The dense mass ofcompacted refractory material also positions and retains in place theterminals 15 and 16.

After the final rolling operation, the assembly is removed from therolling machine and the extreme outer ends of the sheath 11 are strippedso as to remove the backing washers 20, etc., as well as the bent-overends 11:: and 11b of the tubular sheath 11. Finally, the portions of thecompacted refractory material disposed in the extreme outer ends of thesheath 11 and respectively surrounding the terminals 15 and 16 areremoved to provide the end cavities into which the glass plugs or seals17, etc., are ultimately cast so as firmly to retain the terminals 15and 16 in place and so as to seal the opposite ends of the tubularsheath 11, as shown in FIG. 113.

After manufacture of the heating unit .10, it is ordinarily subjected tothe usual electrical tests in order to determine the insulationresistance, proof voltage, heat distribution, and other mattersafiectiug performance and life of the unit.

As a constructional example of the manufacture of the heating unit 10,the tubular sheath 11 may have an initial diameter of 0.312; and in thepreliminary rolling step, the diameter thereof may be reduced to 0.299;and thereafter in the final rolling step, the diameter thereof may bereduced to 0.270". In the heating unit 10, the resistance conductor 12may have a diameter of about 0.0089", the spacing between the centers ofthe turns thereof may be about 0.0249"; and the outside diameter of theturns thereof may be about 0.110". Of course the number of turns perinch longitudinally of the resistance conductor 12 is dependentfoundamentally upon the gauge thereof that, in turn, is dependent uponthe desired wattage rating of the finished heating unit 10. However, asa. practical matter, it has been discovered that from 20 to 42 turns perinch longitudinally of the helix is feasible, when the resistanceconductor 12 is formed respectively of relatively coarse and relativelyfine resistance wire.

in a modification of the methodof making the heating unit 10, theassembly is subjected to heat treatment in the autoclave in the mannerdescribed only throughout a relatively short time interval so as toefiect the conversion of only the outer skin of the magnesium rod 18into amorphous magnesium oxide, leaving the interior core thereof as theoriginal magnesium metal. For example, the outer skin of the member 13may be converted into magnesium oxide, so that the conversion of themember 18 is about complete, by treatment in the autoclave throughout atime interval of about two hours.

After the manufacture of the heating unit 1%), it is incorporated in anappliance, or the like, such, for example, as an electric hotp-late, asillustrated in FIG. 3. Specifically, the inner heating element 30 of ahotplate is ill-ustratcd in FIG. 3; and the heating element 3% is formedfrom the heating unitll) by appropriate bending thereof into therequired configuration followed by flattening of the upper surface ofthe sheath 11, as indicated at 110. This flattening of the upper surfaceor top of the sheath ll, indicated at 110, of the heating element 30,not only accommodates the ready support ofa cooking vessel to be heated,but it also effects further tightening or compacting of the mass ofrefractory material enclosed in the sheath 11, as well as theelimination of any cracks or fissures in the refractory material thatmight be produced therein incident to the bending of the heating unit 10into the desired configuration of the heating element 30.

Referring now to FIGv 2B of the drawing, the electric heating unit 40there illustrated and made in accordance with the method of the presentinvention is fundamentally of the same construction as the heating unit10 described above and is made fundamentally in the manner previouslyexplained; whereby the heating unit 40 comprises the correspondingelements 41, 42, 45, 46, 47, etc. However, in this embodiment thecomposite mass of refractory material enclosed in the sheath 41comprises a central core portion 44 formed of crystalline magnesiumtootsie oxide and an outer surrounding tubular portion 43 formed ofanhydrous amorphous magnesium oxide. This transposition of the positionsof the two portions 43 and 44 of the composite body of refractorymaterial is produced as a consequence of a modification in the assembly,as illustrated in FIG. 2A. More particularly, in producing the assemblyof FIG. 2A from which the heating unit 40 of FIG. 2B is ultimately made,a helical coil 48 formed essentially of magnesium metal is arranged inthe tubular sheath M in a position disposed exteriorly' of the helicalresistance conductor 42 and in surrounding relation therewith. Inproducing the assembly of FIG. 2A the helical coil 48 may be formedessentially of a ribbon of magnesium metal and is slid in place in thetubular sheath 41 preceding the assembly of the resistance conductor 4?,and the terminals 45 and 46 therein. In the arrangement, the helicalcoil 48 may be in direct contact with the adjacent inner surface of thewall of the tubular sheath 41, the helical coil 48 being loosely fitinto the tubular sheath ill; and of course, the resistance conductor 42is placed centrally of the tubular sheath 41.

After the assembly of HG. 2A is thus produced, it is transferred to theloading machine; whereby the finely divided crystalline magnesium oxide44 is charged thereinto and in embedding relation with respect to theresistance conductor 42 and the helical coil it? so as to produce thecentral arrangement of the crystalline magnesium oxide 44, as indicatedin FIG. 2A. Thereafter, the other end of the sheath 41 is closed, in themanner previously explained; whereupon the assembly, as shown in FIG.2A, is subjected to the preliminary rolling step, as previouslydescribed.

Then the assembly is subjected to heat treatment in the autoclave, inthe manner previously explained; whereby the helical coil 48 isconverted from magnesium metal to anhydrous amorphous magnesium oxideproducingthe outer portion 43 of refractory material disposed in contactwith the inner surface of the wall of the tubular sheath 41 andsurrounding the central portion 44 of crystalline magnesium oxide. Stillsubsequently, the assembly is subjected to the final rolling step in therolling machine; and thereafter the sheath 41 is trimmed and the glassplugs 47, etc, are cast in the opposite ends thereof so as to producethe finished heating unit 40, as illustrated in FIG. 2B.

In a modification of the method of making the heating unit 40, theassembly is subjected to heat treatment in the autoclave in the mannerdescribed only throughout a relatively short timeinterval so as toeffect the conversion of only the outer skin of the magnesium helix 48into amorphous magnesium oxide, leaving the interior core thereof as theoriginal magnesium metal. For example, the outer skin of the helix 43may be converted into magnesium oxide, so that the conversion of thehelix 48 is about 10% complete, by treatment in the autoclave throughouta time interval of about two hours.

Of course, the finished heating unit 40 may be appropriately worked inthe manner of the finished heating.

unit 10, described above, in order to produce the heating element 30.

In the manufacture of either of the heating units 10 or 40, it will beunderstood thatwhen the corresponding member 13 or 48 formed ofmagnesium metal is oxidized into anhydrous amorphous magnesium oxide,there is a corresponding expansion of the volume occupied thereby withinthe corresponding sheath 11 or 41, since there, is an expansion of about200% when .a given mass of magnesium metal is converted into acorresponding mass of anhydrous amorphous magnesium oxide by oxidation.This action, of course, producesfurther compacting of the resulting massof refractory materialin the sheath of the heatingunit furthereliminating voids therein and further contributing toward the productionof a dense mass of refractory material in thefinished heatingunitembedcling the resistance conductor and retaining the same in place.This feature is very advantageous, as it will be understood that thecomposite mass of refractory material mentioned not only serves themechanical functions described with respect to holding the resistanceconductor in place in insulated condition withrespect to the surroundingsheath, but it also serves the function of transmitting the heatproduced in the resistance conductor to the sheath for the usefulheating purpose. It is emphasized that the last-mentioned function isvery important, as it not only contributes to eiiiciency of the finishedelectric heating unit, but it also prevents excessive temperatures ofthe resistance conductor, thereby materially contributing to desirablelong life of the heating unit.

Further, in conjunction with the operation of the autoclave, theprocessor is cautioned that the reaction temperature maintained in theheating chamber thereof must not be permitted to rise to the ignitiontemperature of magnesium metal, since it will be apparent that theignition of the magnesium metal will bring about the production of anexceedingly high temperature, with the consequent melting of theadjacent resistance conductor, or even the enclosing sheath of theheating unit undergoing the heat treatment. Fortunately the ignitiontemperature of magnesium metal in gaseous oxygen at a gauge pressure ofabout 70 p.s.i. is well defined at 1085 F., whereby the reactiontemperature of 1075 F. is entirely safe for this step. In thisconnection, it is noted that the ignition temperature of the magnesiummetal is related to the pressure of the atmosphere of gaseous oxygen,the ignition temperature increasing with increasing pressures of thegaseous oxygen atmosphere. Thus it will be appreciated that the reaction'temperature-gauge-pressure relationship mentioned is capable ofappropriae variation dependent upon the factors noted; however, from apractical standpoint, the relatively low gauge pressure of about 70p.s.i. and the readily controllable reaction temperature of about 1075F. are recommended for coinercial production of the heating units inaccordance with the present method.

In the foregoing explanation of the present method, the member 18 of theassembly of FIG. 1A and the member 43 of the assembly of FIG. 2A weredescribed as being formed essentially of magnesium metal; however, amodification is contemplated, wherein these members are formedessentially of beryllium, magnesium, aluminum or titanium, or alloysthereof, as it will be understood that the elements named comprise awell-defined group of metals that may be readily converted from themetallic form into the corresponding metal oxide by oxidation withgaseous oxygen under gauge pressure and at an elevated temperature inthe autoclave in a manner substantially identical to that described, andwherein each of the corresponding metal oxides constitutes a refractorymaterial having good electrical-insulating and good heat-conductingproperties. In each case, the operating temperature of the heatingchamber of the autoclave is established somewhat below the ignitiontemperature of the corresponding metal or alloy so as to preventignition of the metal member in the atmosphere of oxygen gas under gaugepressure at the elevated temperature.

In view of the foregoing, it is apparent that there has been provided animproved and simplified method of making an electric heating unit thatmay be readily carried out upon a mass-production basis for commercialpurposes.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. The method of making an electric heating unit of the sheathedconductor type, which comprises: providing an assembly of an elongatedmember formed essentially of magnesium, an elongated helical resistanceconductor wound upon and supported by said member, an elongated tubularmetallic sheath enclosing both said resistance conductor and said memberand spaced from said resistance conductor, and a porous packing offinely divided electrical-insulating and heat-conducting refractorymaterial arranged in said sheath and embedding both said resistanceconductor and said member and retaining the same in place in said sheathwith said resistance conductor in spaced relation with said sheath;confining said assembly in an autoclave; subjecting said confinedassembly to gaseous oxygen at a gauge pressure so as to contact saidmember and to permeate said porous packing of refractory material withsaid gaseous oxygen; and simultaneously heating said confined assemblyto an elevated temperature disposed below the ignition temperature ofmagnesium metal and throughout a sulficient time interval to convertsaid member into a charge of magnesium oxide while it is thus embeddedin said porous packing of refractory material.

2. The method of making an electric heating unit of the sheathedresistance conductor type, which comprises: providing an assembly of anelongated member formed essentially of magnesium, an elongated helicalresistance conductor Wound upon and supported by said member, anelongated tubular metallic sheath enclosing both said resistanceconductor and said member and spaced from said resistance conductor, anda porous packing of finely divided electrical-insulating andheat-conducting refractory material arranged in said sheath andembedding both said resistance conductor and said member and retainingthe same in place in said sheath with said resistance conductor inspaced relation with said sheath; confining said assembly in anautoclave; subjecting said confined assembly to gaseous oxygen at arange pressure of about 70 p.s.i. so as to contact said member and topermeate said porous packing of refractory material with said gaseousoxygen; and simultaneously heating said confined assembly to an elevatedtemperature disposed below the ignition temperature of magnesium metaland throughout a sufficient time interval to convert said member into acharge of magnesium oxide while it is thus embedded in said porouspacking of refractory material.

3. The method of making an electric heating unit of the sheathedresistance conductor type, which comprises: providing an assembly of anelongated member formed essentially of magnesium, an elongated helicalresistance conductor wound upon and supported by said member, anelongated tubular metallic sheath enclosing both said resistanceconductor and said member and spaced from said resistance conductor, anda porous packing of finely divided electrical-insulating andheat-conducting refractory material arranged in said sheath andembedding both said resistance conductor and said member and retainingthe same in place in said sheath with said resistance conductor inspaced relation with said sheath; confining said assembly in anautoclave; subjecting said confined assembly to gaseous oxygen at agauge pressure so as to contact said member and to permeate said porouspacking of refractory material with said gaseous oxygen; andsimultaneously heating said confined assembly to an elevated temperatureof about 1075" F. throughout a suificient time interval to convert saidmember into a charge of magnesium oxide while it is thus embedded insaid porous packing of refractory material.

4. The method of making an electric heating unit of the sheathedresistance conductor type, which comprises: providing an assembly of anelongated member formed essentially of magnesium, an elongated helicalresistance conductor wound upon and supported by said member, anelongated tubular metallic sheath enclosing both said resistanceconductor and said member and spaced from said resistance conductor, anda porous packing of finely divided electrical-insulating andheat-conducting refractory material arranged in said sheath andembedding both said resistance conductor and said member and retainingthe same in place in said sheath with said resistance conductor inspaced relation With said sheath; confining said assembly in anautoclave; subjecting said confined assembly to gaseous oxygen at agauge pressure of about 70 p.s.i. so as to contact said member and topermeate said porous packing of refractory material With said gaseousoxygen; and simultaneously heating said confined assembly to an elevatedtemperature of about 1075 F. throughout a sufiicient time interval toconvert said member into a charge of magnesium oxide While it is thusembedded in said porous packing of refractory material.

References Cited in the file of this patent UNITED STATES PATENTSWoodson June 10, 1930 Backer May 10, 1932 Deroche May 7, 1940 Heath eta1 Feb. 17, 1942 Dahl Nov. 23, 1948 Thomas Mar. 2, 1954 Huck Feb. 21,1956 Andrews Oct. 28, 1958 Andrews Aug. 11, 1959 FOREIGN PATENTS GreatBritain Jan. 15, 1931 Great Britain 1 Nov. 2, 1942 Great Britain Nov. 3,1948 UNITED STATES-PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, -3OO'Z 2S6 November 7 I961 Emmett W. Barnes It is hereby certified thaterror appears in the above numbered pat ent requiring correction andthat the said Letters Patent should read as corrected below.

Column 7 line 74!; after "sheathed" insert resistance column 8 line 35for "range read gauge Signed and sealed this 1st day of May 19620 (SEAL)Attest:

ERNEST w; SWIDER AVID L. LADD Attesting Officer Commissioner of PatentsUNITED STATESPA'TENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3 OO7236 November "I 1961 Emmett Wa Barnes It is hereby certified that errorappears in the above numbered pat ent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 'Z line 74 after "sheathed" insert resistance -5 column 8 line 35for "range" reed gauge Signed and sealed this 1st day of May 1962,

(SEAL) Attest:

ERNEST w; swmza, I DAVID L. LADD Attesting Officer Commissioner ofPatents

